Reduced ER-mitochondria connectivity promotes neuroblastoma multidrug resistance

Most cancer deaths result from progression of therapy resistant disease, yet our understanding of this phenotype is limited. Cancer therapies generate stress signals that act upon mitochondria to initiate apoptosis. Mitochondria isolated from neuroblastoma cells were exposed to tBid or Bim, death effectors activated by therapeutic stress. Multidrug‐resistant tumor cells obtained from children at relapse had markedly attenuated Bak and Bax oligomerization and cytochrome c release (surrogates for apoptotic commitment) in comparison with patient‐matched tumor cells obtained at diagnosis. Electron microscopy identified reduced ER–mitochondria‐associated membranes (MAMs; ER–mitochondria contacts, ERMCs) in therapy‐resistant cells, and genetically or biochemically reducing MAMs in therapy‐sensitive tumors phenocopied resistance. MAMs serve as platforms to transfer Ca²⁺ and bioactive lipids to mitochondria. Reduced Ca²⁺ transfer was found in some but not all resistant cells, and inhibiting transfer did not attenuate apoptotic signaling. In contrast, reduced ceramide synthesis and transfer was common to resistant cells and its inhibition induced stress resistance. We identify ER–mitochondria‐associated membranes as physiologic regulators of apoptosis via ceramide transfer and uncover a previously unrecognized mechanism for cancer multidrug resistance.

Morphological Pathways of Mitochondrial Division

Mitochondrial fission is essential for distributing cellular energy throughout cells and for isolating damaged regions of the organelle that are targeted for degradation. Excessive fission is associated with the progression of cell death as well. Therefore, this multistep process is tightly regulated and several physiologic cues directly impact mitochondrial division. The double membrane structure of mitochondria complicates this process, and protein factors that drive membrane scission need to coordinate the separation of both the outer and inner mitochondrial membranes. In this review, we discuss studies that characterize distinct morphological changes associated with mitochondrial division. Specifically, coordinated partitioning and pinching of mitochondria have been identified as alternative mechanisms associated with fission. Additionally, we highlight the major protein constituents that drive mitochondrial fission and the role of connections with the endoplasmic reticulum in establishing sites of membrane division. Collectively, we review decades of research that worked to define the molecular framework of mitochondrial fission. Ongoing studies will continue to sort through the complex network of interactions that drive this critical event.